Condenser microphone amplifier



Aug. 23, 1932. F "w 1,872,858

CONDENSER MICROPHONE AMPLIFIER Filed Jilly 22.- 1929 "MlI/YSEfi MICROPHONE INVENTOR.

Wrmess y I) i Patented Aug. 23, 1932 1 UNITED STATES gearenr oFF cE DONALD FAIRFAX wnrrme. or rear WASHINGTON", new YORK, ASSIGNOR 'ro rox case con-rona'rron, or new YORK, n. Y., A oonronarron ornnw YORK v oonnnnsnamonornonn AMP IFIER I Application filed July 22,

This invention relates to a new and improved condenser microphone amplifier.

Condenser microphone amplifiers used in making sound records for talking moving pictures must be compact. The amplifier formerly used employed a grid bias battery as part of the equipment enclosed in the amplifier case. This arrangement proved very unsatisfactory especially when recording in the open under the influence of a hot sun. Consequently the life of the grid battery was short, and frequent renewals had to be made.

Former condenser microphone amplifier circuits experienced frequent grid coupling condenser failures. This condenser was located in series with the central connection on the microphone and the grid of the amplifying tube. Failure was attributed to the method of applying the 200 volt potential necessary for the operation of the condenser microphone.

It is an object of my invention to provide a new condenser microphone amplifier that does not require a separate grid bias battery. The new circuit provides grid bias for the tube without the aid of a separate battery.

It is a further object of my invention to provide a condenser microphone amplifier circuit that will reduce condenser failure in the circuit to a minimum.

Other objects and advantages will be apparent with reference to the accompanying drawing in which:

The circuit diagram ofthe new amplifier is shown. Condenser microphone -1 is connected to the amplifier circuit at 2 and 3. From the center microphone connection 3 lead 15'- connects to grid circuit condenser 5 which, in turn, connects to the grid of the vacuum tube 7. Connection from the frame of the microphone is made at 2 through wire 16- to condenser 10 through wire 22, which connects to the negative filament terminal of the tube 7.

The microphone battery makes connection with the amplifier at the plus 200 V terminal. Wire 23 connects the microphone battery with a high resistance 8- which connects 1929." Serial no. 380,199,

to'wire 16 at -19-. Byconnecting the high voltage to the frame of the microphone -1 it is necessary to insulate it from its support; Coupling condenser ''10- also serves as ablocking condenser keeping the high microphone voltage out of the filament circuit of the tube -7. Coupling resistance Insulating the condenser microphone.

1- from its support anden-erg'izing the frame/side by means of a high voltage battery the grid coupling condenser -5 is freefrom high voltage effects. Leakage due to high voltage across the 'condenser' is id" leak resistance -'6-- connection through I thereby eliminated. The only possibility for high voltage to get in connection with condenser 5- will be during microphone failure. A leaky, microphone Will allow the high voltage to pass through to 'the'condenser, but since a leaky microphone would not be operative anyway, this constitute'sfno disadvantage. Eliminating leakage through the grid condenser prevents any currents from flowing through resistance"6'- thereby changing the grid bias of the tube.

' Connecting the grid of tube 7-"through resistances 6 and 9 to the negative filament terminal 6V makes the grid negative with respect to the filament. A grid bias is thereby obtained without the aid of a grid bias battery. Eliminating the battery also provides a great saving in space required for the amplifier. V

For the purpose of illustration, I have used a specific amplifier, however, I do not desire to be restricted by the use of the particular' apparatus shown, as many changes and modifications can be made within the scope of the appended'claims.

I claim: 1. The comblnatlon with a condenser microphone of an amplifier comprising a Vacuum tube, a grid circuit connected through a coupling condenser to the central terminal of said microphone, a filament circuit, the

negative side of the same being connected through a condenser to the frame side of said microphone, a resistance connecting the central microphone terminal to one side of filament of sai'd' tube',-a filament battery, agrid leak' resistance connecting the grid of said tube with the negative terminal of said bat- J tery, a B-battery, meansflfor making connec-v tion from said B-battery to the plate of said tube, means for connecting the. negative tersaid tube, a resistance connection from the central terminal of said microphone to one hand this 15th day of July, 1929.

DONALD FAIRFAX WHI'lING.

minal of saidB battery to the negative terminal of said filament battery and means for L connecting the positive terminal of said B- battery' to a resistance, said resistance, being connected to the frame slde of sald condenser microphone.

2. The combinationwith a condenser microphone, of an amplifier comprising a'vacuum tube, the grid circuit of which is connected through a coupling condenser-to the central terminal of said microphone, a filament circuit, the negative side of which is connected through a condenser to the frame side of said IIllCIOQllOIle, a resistance connected at one end to the central microphone terminal and at; the other to one side of the filament of said tube, a source of filament potential, a grid resistance connecting the grid of said tube with the negative side of said filament potential hereby the' grid is maintained at a negative potetnial with respect to the filament, ahigh voltage source 7 of direct current, a connection from thepositive side of said high voltage source to the plate electrode of said tube, a connection from the negative-terminal of said high voltage source to the negative terminalof said source of filament potential and'a connection 7 from the positive side of said high voltage source to the frame terminal of said microph n -i ,7 r 3; In the combination with a condenser microphone, of an amplifier having a vacuum' tube and filament and plate potential sources therefor connected in operative relation, a capacity connection from the central terminal of saidvmicrophone to the grid of said tube a resistance connection from the v central terminal of said microphone to one side of the'filament of said tube, and means for connecting the frame of said microphone tothe positive terminal of said plate potential source. 4 v

4.:In the combination with a condenser microphone, of an amplifier having a vacuum tube and filament and plate potential sources therefor connected in operative relation, a capacity connection from the central terminal of saidrnicrophone to the gridof 75 In Witness whereof I have'hereuntoset my- 

